1. Field of the Invention
Embodiments of the present invention generally relate to optical fibers and, more particularly, to preforms for making polarization controlling optical fiber and methods for fabricating such preforms.
2. Description of the Related Art
Optical fibers are typically formed by heating and drawing an optical fiber preform. The preform typically includes a core and surrounding cladding, with appropriate dopants to achieve desired characteristics. In an effort to control polarization (e.g., maintain polarization or achieve a desired polarization) of light transmitted through the core of the resulting optical fiber, the preform often includes stress-inducing members.
Traditionally, polarization maintaining (PM) fibers have been designed incorporating diametrically opposed elements that induce asymmetric radial stresses on the fiber core, thereby creating birefringence in the core. These stress elements are typically silica glass that is heavily doped with boron, germania, and/or phosphorous oxides, which greatly increase the glass thermal expansion coefficient. Examples of fibers with typical stress elements placed around a core 102 and cladding 104 are illustrated in FIG. 1. These include a fiber 100 with an elliptical shaped element 106, a fiber 120 with two circular rods 126 (PANDA), and a fiber 130 with a ‘bow-tie’ element 136.
The preform fabrication processes differ for each of these designs. The elliptical and bow-tie design preforms are manufactured using the modified chemical vapor deposition (MCVD) method, where all of the glass components are deposited within a glass tube. The elliptical design fiber is achieved by grinding parallel sides on the preform prior to drawing. The bow-tie design is achieved by etch and deposition methods inside the tube during the MCVD process. The MCVD process is limited by the amount of glass that can practically be deposited inside of a tube. Also, as the tube wall becomes heavier with deposited glass, the final collapse step of the MCVD process becomes much more difficult. The ‘PANDA’ design involves manufacture of an optical preform via traditional MCVD, Vapor-phase Axial Deposition (VAD), or Outside Vapor Deposition (OVD) methods and then drilling holes longitudinally to insert the glass stress elements prior to fiber drawing. This method is limited by the ability to machine precise, long length-wise holes in the host preform.
In depressed refractive index fiber designs, such as with a pure silica glass core fiber, it is desirable to have a large optical clad to core ratio (˜>6) to reduce bend-induced waveguide losses. The clad to core ratio may be decreased by increasing the core to clad refractive index difference, however the fiber's core size may become too small for practical use. Unfortunately, conventional PM fiber preform designs described above are limited to the type of glass that may be deposited with vapor deposition methods. Further, these designs typically produce preforms of small volume, which yield relatively small batches of optical fiber.
Accordingly, what are needed are improved methods to efficiently manufacture preforms to produce polarization controlling fibers.